This invention relates to an optical fiber composite overhead transmission line cable obtained by providing an optical fiber cable unit inside an electric aerial transmission line formed by stranding a plurality of electrical conductors, or in an overhead earth (ground) wire which is extended in parallel with such an overhead transmission line.
Optical fiber cables have been extensively used as signal transmission media, and have specific features in that not only are they large in data transmission capacity, but they are also free from electromagnetic induction. Accordingly, an optical fiber cable or unit can be incorporated in an overhead transmission line or an ordinary power line. That is, the optical fiber cable provides a great economical effect in that it is unnecessary to additionally install a separate optical communication cable.
Heretofore, in order to incorporate the optical fiber cable into an overhead electrical transmission line, some of the outer stranded electrical conductors forming the overhead transmission line are replaced by an optical fiber cable unit (hereinafter referred to as "an optical fiber unit") which is obtained by incorporating a plurality of optical fibers in a metal tube. In the above-described conventional construction, the wall thickness of the metal tube covering the optical fiber unit is generally small because of dimensional limitations. Accordingly, the metal tube is insufficient in mechanical strength and accordingly cannot sufficiently protect the optical fibers from damage. Thus, the outer cover is liable to be damaged and water may enter the optical fiber unit.
On the other hand, since it is difficult to increase the inside diameter of the metal tube, it is considerably difficult to increase the percentage of twist of the optical fiber cable itself, and accordingly it is impossible to form a cable with the optical fibers slackened.